(Not Applicable)
(Not Applicable)
The present invention generally relates to temperature compensation circuitry and more specifically to an application specific integrated circuit (ASIC) temperature compensation circuit for use with voltage controlled attenuators or variable gain amplifiers for the purpose of compensating gain variations over temperature of radio frequency (RF) amplifiers.
Uncompensated RF amplifiers typically have inherent and somewhat predictable gain variations over operating temperatures due to changes in the transconductance of the active devices. The change in amplifier gain varies inversely with the operating temperature (i.e., as temperature increases, gain decreases) and can vary at a rate of between 0.012 to 0.02 dB/xc2x0C. for an amplifier with approximately 10 dB of RF gain. The variations in amplifier gain over temperature results in reduced sensitivity and dynamic range for many systems unless adequately compensated. Typically, voltage variable attenuation devices have been used to compensate for gain variations of amplifiers. Alternatively, another technique commonly used is to apply an adjustable bias, relative to operating temperature, to the amplifier itself, which results in the necessary gain compensation to maintain the overall amplifier gain at a relatively constant level.
Prior art temperature compensation circuitry used for the purpose of temperature compensating amplifiers consisted of thermistors and temperature references that generate non-linear drive characteristics for the voltage variable attenuation or amplifier device. These prior art temperature compensation circuits allow the gain variation of the amplifier caused by temperature to be compensated out of the system by appropriately varying the system attenuation via bias voltages. However, the prior art temperature compensation circuitry has been inadequate because it is costly, consumes large amounts of substrate area, and only approximates the desired response. Additionally, the prior art temperature compensation circuitry is inadequate because a new or modified circuit design is required for each application and for each variation in amplifier fabrication.
The present invention addresses the above-mentioned deficiencies in prior art temperature compensation circuitry by providing a circuit which accurately provides the required control voltage to precisely compensate for variations that result from temperature changes. The present invention addresses a technique for generating the required input bias control to a wide variety of voltage controlled gain or attenuator devices for the purpose of accurately adjusting and compensating for gain variations of amplifiers over temperature. This same circuit described herein can be used to temperature compensate other devices (i.e., oscillators, attenuators, phase shifters, etc.) that are subject to variations over temperature. The present invention provides for application specific temperature compensation responses by providing multiple (2 or more), independent precise voltage outputs. Many voltage variable attenuators require multiple voltage inputs that are non-linear and independent (i.e., can not be derived from each other). The present invention is not limited to only 2 outputs and can be expanded to accommodate 3 or more precise outputs that can be adjusted to provide unique voltage output versus temperature transfer curves for each output. Additionally, the present invention provides a temperature compensation circuit that can be fabricated on an Application Specific Integrated Circuit (ASIC), thereby making the circuit less expensive and more compact in size than the prior art.
A universal temperature compensation circuit for use with a temperature sensor operative to generate an analog temperature voltage based on the temperature thereof. The universal temperature compensation circuit comprises an analog to digital converter in electrical communication with the temperature sensor. The analog to digital converter is operative to generate a digital temperature voltage from the analog temperature voltage. In electrical communication with the analog to digital converter is an electronic storage device containing digital values or data correlated to temperatures of the temperature sensor. The universal temperature compensation circuit further includes a digital to analog converter in electrical communication with the electronic storage device. The digital to analog converter is operative to generate an analog voltage level from the digital values or data contained within the electronic storage device. An amplifier is in electrical communication with the digital to analog converter and is operative to amplify and buffer an output voltage level from the analog voltage level. The output voltage level is correlated to the temperature of the temperature sensor by the digital data contained within the electronic storage device. It is contemplated that a demultiplexer be electrically connected between the electronic storage device and the digital to analog converter. In this instance, a second digital to analog converter may be in electrical communication with the demultiplexer and a second amplifier. Accordingly, the second amplifier will be operative to output a second output voltage level that is correlated to the temperature of the sensor but is different than the first output voltage level.
It will be recognized that the electronic storage device may be an EEPROM that is programmed with temperature specific data. The digital temperature signal is used as an address for the EEPROM and addresses the digital value or data stored in the EEPROM that corresponds to the temperature of the sensor.
In order to control the operation of the temperature compensation circuit, there is included a timing circuit. The timing circuit is operative to control the operation of the EEPROM, demultiplexer, and perform temperature sampling in order to output the correct digital data at the correct time. The timing circuit includes a comparator to control the operation of the temperature compensation circuit such that a clock is not required to operate 100% of the time. In the preferred embodiment of the present invention, the temperature compensation circuit is fabricated as an application specific integrated circuit on a single chip.
The temperature compensation circuit is primarily used to drive the control inputs for a voltage variable attenuator located in a RF amplifier chain. In this respect, the electronic storage device is programmed with temperature specific data used to control the attenuator. As will be recognized, the electronic storage device may-be programmed with other temperature specific data in order to control other types of devices.
In accordance with the present invention there is provided a method of correlating a temperature found with a temperature sensor to a prescribed voltage level using the universal temperature compensation circuit. The method comprises the step of generating an analog temperature voltage with the temperature sensor. Next, the analog temperature voltage is converted to a digital temperature signal with the analog to digital converter. Digital data corresponding to the temperature of the temperature sensor is generated by the electronic storage device from the digital temperature signal. Next, the digital data is converted to an analog voltage level with the digital to analog converter. Finally, the analog voltage level is buffered and amplified to generate an output voltage correlated to the temperature of the temperature sensor.
In the preferred embodiment of the present invention, the digital value or data stored within the electronic storage device is generated by addressing the digital value or data with the digital temperature signal. Typically, the electronic storage device is an EEPROM and the digital temperature signal is used to address the EEPROM. If the digital compensation circuit includes a demultiplexer, a second digital to analog converter, and a second amplifier, then the demultiplexer will direct the digital data level to the second digital to analog converter. In this respect, a second final output voltage will be generated by the second amplifier. The second final output voltage may not be equal to the first final output voltage. In the same manner as the second voltage was generated, additional voltages can be generated to control other components that are subject to temperature fluctuations.